Reliefs, especially flexographic relief printing plates useful for letterpress printing, can be prepared from photopolymerizable elements comprising (1) a layer of solvent-soluble photosensitive elastomeric composition containing (a) a thermoplastic, elastomeric polymeric binder comprising polymerized conjugated diene monomers, (b) an addition polymerizable, nongaseous, ethylenically unsaturated compound, and (c) an addition polymerization initiator or initiator system activatable by actinic radiation; (2) a permanently affixed support; and, usually, (3) a removable cover sheet.
Such elements and processes for their use in preparing relief printing plates are well known in the art; see, e.g., Plambeck, U.S. Pat. No. 2,760,863; Suzuki et al., U.S. Pat. No. 3,556,791; Varga et al., U.S. Pat. No. 3,798,035; Kurka, U.S. Pat. No. 3,825,430; Recchia et al., U.S. Pat. No. 3,951,657. It is often desirable to interpose a flexible, transparent polymeric film between the cover sheet and the surface of the photosensitive layer to protect the transparency used in imaging the element. The thickness of the photosensitive layer used depends on the thickness desired in the relief image. In general, the thickness of the photopolymerizable layer will vary from about 0.005 to about 0.250 inches or more and layers within this thickness range will be used for the majority of printing plate applications.
In general, the process of preparing a flexographic printing plate from a photopolymer element includes the steps of "backflash", main image exposure, development or "washout", drying, detackification, and post-exposure.
The "backflash" exposure may be used with elements having a transparent support. Backflash generally uses a radiation source emitting a principal wavelength around 360 nm. It serves to sensitize the plate and establishes the depth of the plate relief. The backflash exposure gives the photopolymer film a uniform and relatively short exposure through the support, thereby photocrosslinking binder and monomer in the floor region.
The coversheet is then removed and an image-bearing transparency is placed on the photopolymer surface or, preferably, on a transparent flexible protective polymeric film overlaying the photopolymer layer. The main imagewise exposure also generally utilizes a source emitting strongly at around 360 nm (340 to 400 nm), which photocrosslinks binder and monomer, creating insolublized areas extending from the plate surface to the floor created by the backflash exposure.
After these exposures, the photopolymerizable composition is removed in the unexposed areas by treatment with a suitable solvent which will dissolve the unexposed areas of the layer but not the exposed, polymerized areas. This step is known as development or "washout". Solvent development may be carried out at about 25.degree. C., but best results are sometimes obtained when the solvent is warm, e.g., 30.degree. to 60.degree. C. Development time can be varied, but is preferably in the range of about 5 to 25 minutes. Developer may be applied in any convenient manner, including immersion, spraying, and brush or roller application. Brushing aids in removing the unpolymerized or non-crosslinked portions of the composition. Washout is frequently carried out in an automatic processing unit which uses solvent and mechanical brushing action to remove the unexposed portions of the plate, leaving a relief constituting the exposed image and floor.
Following solvent development, the relief printing plates are generally blotted or wiped dry, and then dried in a forced air or infrared oven. Drying times and temperatures vary, but drying for 60 to 120 minutes at 60.degree. C. (140.degree. F.) is typical. High temperatures are not recommended as shrinkage of the support may cause registration problems. Additional air drying overnight (sixteen hours or more) is common. Solvent will continue to evaporate from the printing relief during drying at ambient conditions.
Even after drying, however, photopolymer flexographic printing plates generally retain at least some degree of surface tackiness, particularly on the "shoulders" of the relief and other non-image areas. This surface tackiness is undesirable in a printing relief. Not only is it difficult to handle a tacky printing relief, but such plates have a tendency to stick together when temporarily stacked for storage. Moreover, tacky plates pick up ordinary dust and dirt, as well as paper dusts when used for printing on paper. It is common practice, therefore, to employ one of several currently known detackification or "finishing" measures. For example, it is well known in the art that chemical treatment with either dilute aqueous chlorine or bromine, or exposure to short wavelength irradiation, can reduce this surface tackiness. All of these measures, however, have various limitations or shortcomings.
In addition to "finishing" to remove surface tackiness, most flexographic printing plates are uniformly post-exposed to ensure that the photocrosslinking process is complete and that the plate will remain stable during printing and storage. This "post-exposure" utilizes the same ultraviolet radiation source as the main exposure (usually wavelengths of 300 to 420 nm). Post-exposure is used to complete polymerization and maximize plate hardness and durability, but does not remove tackiness. Accordingly, both detackification and post-exposure procedures are routinely carried out.
A variety of detackification processes utilize some form of chlorine or bromine. See, e.g., U.S. Pat. No. 4,400,460 and German Patent, DE No. 2 823 300. U.S. Pat. No. 4,400,459 to Gruetzmacher et al. teaches a process for detackifying photosensitive elastomeric flexographic printing plates, wherein the developed, dried surface is, in either order, (1) post-exposed to actinic radiation, and (2) treated with a solution of an alkali monopersulfate and a bromide salt.
U.S.S.R. Pat. No. 542 167, Inventors: Shur et al., teaches a method of producing photopolymer printing plates, wherein after development, the printing plate is reexposed under a layer of protective fluid, thereby eliminating tackiness. Fluids which do not react with or damage the polymerized material may be used, including water. When using radiation sources that produce short wavelength UV-radiation (less than 250 nm), solutions of mineral salts (KNO.sub.3 CuSO.sub.4, etc.) as well as dyes may be used as the protective fluid to separate the actinic zones from other zones of the radiation.
U.S. Pat. No. 4,202,696 to Takahashi et al. teaches a method of removing surface tack from photopolymer printing plates by impregnating the surface layer with an organic carbonyl compound capable of abstracting a hydrogen atom and then irradiating the plate with actinic radiation having wavelengths of 200 to 300 nm to excite the organic carbonyl compound. Significant amounts of this wavelength radiation can be obtained from germicidal lamps, high pressure mercury lamps, low pressure mercury lamps, and heavy hydrogen lamps.
U.S. Pat. No. 3,506,440 to Sugimoto teaches a method of reinforcing exposed and developed photosensitive coatings containing principally polyvinyl cinnamate by post-exposing to ultraviolet radiation having a wavelength range of 2200 to 3400 Angstroms (220 to 340 nm). A sterilizing lamp which produces a strong spectrum of light of 2537 A. wavelength is said to be particularly preferred.
European Published Patent Application No. 0 017 927 to Nakamura et al. teaches a process for detackification of photosensitive elastomeric printing plates by exposing to light having a wavelength not longer than 300 nm, e.g., with "germicidal" lamps. While the described process provides an effective and convenient means of detackifying a photopolymer printing plate which has been developed or "washed out" with an organic solvent or mixture of solvents, e.g., perchloroethylene/n-butanol or trichloroethylene, it has been found that this process does not provide satisfactory detackification of a photopolymer plate which has been developed in an aqueous or semiaqueous solution. It has been found, moreover, that some residual aprotic organic solvent must be present for satisfactory detackification to occur, i.e., overdrying of photopolymer relief plates developed in solutions such as perchloroethylene/n-butanol results in unpredictable and unsatisfactory detackification with short wavelength radiation such as that emitted by "germicidal lamps".
Accordingly, the present invention relates to an improved method of detackifying photopolymer printing reliefs which have been processed in an aqueous or semiaqueous developer. It also provides a method whereby organic solvent-processed photopolymer printing reliefs can be detackified with greater predictability and uniformity using germicidal radiation, and whereby such a method can be used with overdried organic solvent-processed printing reliefs.